The use of DC--DC power converters is wide spread in many important industries including those associated with larger telecommunication and computer installations. These units are often expected to operate reliably over a variety of load and temperature conditions. These converters transform an input DC is voltage by converting it first to an AC signal, typically passing it through a transformer and then rectifying it to provide the desired value of output DC voltage. The use of switching techniques allows the power density of these converters to be increased.
A major problem area with high frequency, high power DC--DC converters revolves around a reverse recovery current condition associated with the power rectifiers that occurs during a turn-off transition and the resulting switching losses associated with this effect. High frequency operation of the switching devices of the DC--DC converter allows the use of smaller energy storage elements and filtering components (such as transformers, inductors and capacitors) in the converter. As the switching frequency of the switching devices is pushed even higher to increase the converter power density, the reverse recovery condition associated with the power rectifiers becomes more severe. A significant reverse recovery current may, at worst, damage or destroy the power rectifiers and, at best, contribute to poor power conversion efficiency.
Other problems arise when high blocking voltage rated switching devices are required. The cost of the high blocking voltage rated switching devices is much higher than the lower voltage rated switching devices. Additionally, the higher voltage rated devices exhibit higher forward conduction voltage drops than the lower voltage rated devices which makes them more lossy and therefore less efficient overall.
To deal with these problems, various passive and active snubber circuits have been developed to address and compensate for these undesirable qualities. Some of these snubber circuits are very complicated and difficult to implement. Many have high losses themselves and therefore contribute to lower converter efficiency which, while offering protection to the power rectifiers, just transfers much of the overall power loss to the snubber circuit.
Among the snubber circuits developed, the energy recovery snubber circuit with reduced turn-off loss is one of the more attractive operationally. The energy recovery snubber circuit, however, may require several additional circuit components which often makes the circuit layout challenging in terms of minimizing stray inductance. Stray inductance causes spurious "ringing" at switching transition times which often significantly increases the voltage stresses on the power rectifiers if left uncompensated. Furthermore, limiting the diode reverse recovery current too severely will not allow the circuit to function properly.
Another snubber circuit design is the resonant, passive snubber circuit. This snubber circuit was developed preferably for transformer isolated converters to protect the power rectifiers from excessive voltage stress produced from the energy stored in the leakage inductance of the power transformer during a power rectifier turn-off transition.
Most DC--DC converters are expected to operate properly over widely varying values of load current. Many DC--DC converters are designed to supply up to a rated value of load current while maintaining a highly regulated voltage output. As the load current increases beyond this value, the output voltage decreases up to some point at which the output voltage is driven to zero to protect the converter from heat induced failure. Passive snubber circuits, however, which are designed to protect the power rectifiers from too large a reverse voltage, may no longer operate properly as the output voltage decreases causing permanent component failures especially of the power rectifiers.
Accordingly, what is needed in the art is a way to isolate critical snubber circuit components from the effects of an operating converter output voltage that may become too low to maintain proper snubber circuit operation.